Gallium nitride has been drawing attention as a luminescent layer of a blue light emitting diode (LED) and a material of a blue laser diode (LD). In recent years, gallium nitride has been used in the form of a thin film or a substrate in a variety of applications such as white LEDs and blue LDs and has also drawn attention as a prospective material to be used in applications such as power devices. At present a gallium nitride thin film is commonly produced by a metal organic chemical vapor deposition (MOCVD) method. In the MOCVD method, a vapor of a material is incorporated in a carrier gas and transported to the substrate surface where the material is decomposed by a reaction with the heated substrate to allow crystal growth.
Examples of a method of preparing a thin film other than the MOCVD method include a sputtering method. In a sputtering method, a cation such as Ar ion is physically collided with a target provided on cathode to allow the material constituting the target to be emitted by the collision energy to deposit a film having substantially the same composition as the target material onto a substrate provided on the opposite side. Examples of such sputtering method include direct-current sputtering method (DC sputtering method) and radio-frequency sputtering method (RF sputtering method).
Conventionally, a metal gallium target has been used in a method of preparing a gallium nitride thin film by sputtering (see, for example, Patent Document 1). However, in such cases where a metal gallium target is used, the target may be melted because the melting point of metal gallium is about 29.8° C.; therefore, it is necessary that an expensive cooling device is installed and the film formation is performed at a low power in order to prevent the target from melting, and there is a problem of reduced productivity.
Further, there has been proposed a gallium nitride thin film prepared by using a sputtering target containing gallium nitride as a main component (see, for example, Patent Document 2); however, the density and physical properties of such gallium nitride target have not been examined. In addition, a system in which Tb or the like is added has been prepared as a sputtering target; however, no examination has been made on a system consisting of only gallium nitride or nitrogen and gallium.
Moreover, there has been proposed a high-density sintered body of gallium nitride (see, for example, Patent Document 3). Such a sintered body is densified at a pressure of 58 Kbar (5.8 GPa); however, an apparatus required for applying such a pressure is very expensive and is, therefore, not suitable for preparing a large-sized sintered body.